The hydrophilic colloids produced by Xanthomonas species are polysaccharides which contain mannose, glucose, glucuronic acid, O-acetal radicals and acetal-linked pyruvic acid. These gums and their derivatives have found wide food and industrial applications. Of special interest is the increasing focus on the use of Xanthomonas gums in displacement of oil from partially depleted reservoirs.
Typically, oil is recovered from underground reservoirs via a series of sequential operations. A new well will generally produce a limited amount of oil as a result of release of internal pressure in the well. As this pressure becomes depleted, it is necessary to pump further quantities of oil by mechanical means. These measures recover only about 25% or less of the total oil stored in the reservoir. A great deal of oil is still trapped within the pores of the formation. Further enhancement of recovery can then be effected by secondary methods. In one method of recovery, a waterflood is carried out by pumping water into a well or series of wells, displacing part of the trapped oil from the porous rock and collecting the displaced oil from surrounding wells. However, waterflooding still leaves about 55-60% of the available oil trapped in the formation. The explanation for this is that the water has a very low viscosity compared to the crude oil and tends to follow the path of least resistance, fingering through the oil and leaving large pockets untouched. In addition, surface forces in the formation tend to bind the oil and prevent its displacement.
A number of processes have been developed in recent years to recover further quantities of oil from these reservoirs by the use of mobility control solutions which enhance oil displacement by increasing the viscosity of the displacing fluid, and as a consequence, the mobility of reservoir oil. Of interest are those enhanced recovery processes employing polymer flooding with a polysaccharide or polyacrylamide to increase the viscosity of the displacing fluid. Variations of this process include the use of surfactants and co-surfactants to release the oil from the rock formation. Polyacrylamides have been found to suffer such deficiencies as viscosity loss in brines and severe shear sensitivity. Since, as was well documented in the prior art, xantham gum is relatively insensitive to salts (does not precipitate or lose viscosity under normal conditions), is shear stable, thermostable and viscosity stable over a wide pH range, xanthan gum is a good displacing agent. Moreover, the gum is not extensively adsorbed on the elements of the porous rock formations and it gives viscosities useful in enhanced oil recovery (5 to 100 centipoise units at 7.3 sec..sup.-1 shear rate) at low concentrations (100 to 3000 ppm). The use of solutions of xanthan gum or derivatives of xanthan gum for oil recovery is described in U.S. Pat. Nos. 3,243,000; 3,198,268; 3,532,166; 3,305,016; 3,251,417; 3,319,606; 3,319,715; 3,373,810; 3,434,542; 3,729,460 and 4,119,546. It is suggested in U.S. Pat. No. 3,305,016 that aqueous solutions containing heteropolysaccharide in sufficient quantity to increase the viscosity be employed as the thickening agent in preparing viscous waterflooding solutions. The polysaccharide may be prepared, separated, purified and then added. Alternatively, according to this reference, the entire culture after adding a bactericide (e.g., formaldehyde) to kill the bacteria, may be added to the flood water.
It has been found that various heat treatments result in enhanced viscosities or filterability of whole and diluted Xanthomonas fermentation broths. U.S. Pat. No. 3,501,578 provides that a heat step is carried out prior to the precipitation of xanthan. Viscosity increases of 1.5 to 3.5 fold are obtained in the heat-treated broth. U.S. Pat. No. 3,773,752 describes a process for heating diluted fermentation broth after addition of an alkali metal salt until coagulation occurs and filtering the hot solution preferably after the addition of a coagulating agent such as alum. The process of U.S. Pat. No. 3,801,502 calls for the addition of an alcohol, phenol, ketone or non-ionic surfactant during the heating process. In the process of U.S. Pat. No. 3,355,447, the heat-treated fermentation broth is diluted, filtered and the xanthan removed by alcohol precipitation.
In spite of this advanced state of technology, a difficult problem remained unsolved; i.e. xanthan biopolymer has carboxyl groups which can serve as cross-linking sites for polyvalent metal ions such as iron, magnesium and calcium. These metal ions are commonly found in oil-bearing formation waters. The result of this crosslinking is biopolymer immobilization and formation plugging due to a gelation mechanism. Oil production is thus reduced because the xanthan cannot readily migrate through the rock formation. Cell matter from the Xanthomonas organism is present to varying extents in xanthan gum. This material also tends to plug the formation. In broth forms of the gum, native cells plug to a much lesser degree. As polyvalent ions such as iron and calcium increase in concentration in field brine, the native cells can plug. It is believed that this is due to cross-linking of xanthan on the surface of the cells which has not yet been released into solution. It is the present invention that for the first time presents a solution to these economically significant problems.